1. Field of the Invention
This invention relates to an ultrasonic motor in which electrical energy is supplied to an electro-mechanical energy conversion element provided on a bar-like resilient member to thereby vibrate the resilient member as a bar-like vibration member and cause circular or elliptical motion in the surface particles of the vibration member, thus frictionally driving a movable member pressed against the vibration member, and particularly to an ultrasonic motor suitable for an optical instrument such as a camera or a business machine such as a printer.
2. Related Background Art
An ultrasonic motor of the type in which a flexural vibration is caused in a circular ring-like resilient member and a lens driving movable member is driven by a frictional force has heretofore been put into practice in an AF mechanism for a camera or the like. However, the motor of this conventional type is of a ring-like shape and therefore is relatively high in cost as a unit including a pressing mechanism, and is disadvantageous in cost as a motor which is not required to be hollow. So, a motor as the type as shown in FIGS. 2 and 3 of the accompanying drawings which is of the solid type and in which the construction of a pressing system or the like is easy has been proposed in recent years.
This proposed motor will now be briefly described with reference to FIGS. 2 to 4 of the accompanying drawings.
FIG. 2 is a pictorial view of a bar-like ultrasonic motor, FIG. 3 is a cross-sectional view of the central portion of the motor shown in Figure 2, and FIG. 4 is a schematic view showing the vibrating of the vibration member of the motor shown in FIG. 2. The .gamma. direction in FIG. 4 refers to a direction along the .gamma. axis perpendicular to the Z direction shown in FIG. 2 (a direction along the axis of vibration members b1 and b2, i.e., the Z axis), and the .gamma.-Z plane refers to a plane formed by the .gamma. axis and the Z axis. The bar-like ultrasonic motor as shown in FIG. 2 has two hollow disk-like piezoelectric elements (for example, PZT) al and a2 interposed between a metallic hollow upper vibration member b1 and a metallic hollow lower vibration member b2, threaded portions bb1 and bb2 provided on the inner periphery sides of the upper vibration member b1 and the lower vibration member b2, respectively, and a bolt c threadably engaged with said threaded portions with the two piezoelectric elements a1 and a2 held therebetween.
When AC signals which are electrically out of phase with each other (usually signals which are 90.degree. out of phase with each other) are applied from a conventional driving circuit, not shown, to the piezoelectric elements al and a2, for example, vibrations which are 90.degree. out of phase with each other in position and in time are excited in the vibration members b1 and b2 because the piezoelectric elements al and a2 as electro-mechanical energy conversion elements are out of phase with each other in polarized position, and the surface particles of the vibration members make rotational movement as viewed from the direction of the Z axis.
The letter d designates a hollow rotor having a projection dd and a contact portion AA which are in contact with the vibration member b1, and the letter f denotes a pressing member for bringing the rotor d into frictional engagement with the vibration member b1 by a spring e through a bearing g.
In such a motor, no contrivance has been made in the portion of contact A between the vibration members b1, b2 and the rotor d and therefore, it has been impossible for the rotor d to always keep contact with the vibration member bl over the entire circumference of the vibration member in which the displacement of vibration is several microns.
Accordingly, it has been impossible to take out the motor output sufficiently and the efficiency of the motor has been bad.
Also, the failure in making smooth contact has given birth to harsh sounds.
Further, there has arisen a problem which will hereinafter be described. Careful observation of the portion of contact A between the vibration member b1 and the rotor d as a movable member during the vibration of the vibration member b1 shows that depending on the ratio between the vibration components in the .gamma. direction and the Z direction, the vibration member b1 has an angle of vibration.
This angle of vibration is defined as an angle .theta. formed with respect to the Z axis, as shown in FIG. 5A of the accompanying drawings.
On the other hand, the direction in which the movable member d is deformed by being subjected to the vibration from the vibration member b1 is determined by the shape of the movable member d, and in the movable member d shown in FIG. 6 of the accompanying drawings, it is the direction of an angle .phi.. Here, the arrow F.phi. indicates the direction of deformation of the movable member d, and Z indicates the aforementioned Z axis.
At this time, as shown in FIG. 7 of the accompanying drawings, the contact portion AA on the movable member d side slides by .DELTA.l (see FIG. 7), and if at this time, sliding takes place, an energy loss will be caused and a reduction in efficiency will be caused.
In FIG. 5A, F1-F4 indicate the directions of vibration at various points on the portion of contact A on the vibration member bl, and the characteristics of FIGS. 5B and 5C show the vibration amplitude of the vibration member b1 in the .gamma. axis direction and the vibration amplitude of the vibration member b1 in the Z axis direction, respectively, during vibration.